In general, polyolefinic thermoplastic elastomers, such as those where polypropylene is the matrix and cured rubber forms a discrete second phase (e.g., SANTOPRENE supplied by Monsanto), adhere poorly to metal, glass, and other substrates. When these types of elastomers are molded against a substrate, such as a metal insert in injection molding, an adhesive must first be applied to the substrate to ensure sufficient adhesion between the elastomer and the substrate.
Chlorinated polyolefins and modified chlorinated polyolefins are well known to provide adhesion to polyolefinic materials, such as polyolefinic thermoplastic elastomers and polypropylene (Eastman Chemicals Publication No. GN-360-C, August, 1988; Eastman Chemicals Publication No. X-294, March, 1989). The influence of the amount of chlorine, the molecular weight of the polymer, its melting point, and other variables on the efficacy of adhesion between chlorinated polypropylene and polyolefinic substrates has been reported by Fujimoto (F. Fujimoto, Paint and Resin, February, 1986, p. 36.). Chlorinated polyolefins are, for example, the primary ingredient in primers used to bond to polyolefins (Renout, European Patent Application 0 187 171, July 1986).
Chlorinated polyolefins do not, however, strongly adhere to other materials such as metal. Eckhardt (U.S. Pat. No. 3,620,860) describes chlorinated polymers of ethylene that are claimed to be effective temporary bonding agents for metals, but the substrates bonded by those chlorinated polymers are readily separated from each other. In order to attain adhesion between compositions containing chlorinated polyolefins and other materials, the chlorinated polyolefins must typically be mixed with other ingredients. Van Meesche and Radar (A. Van Meesche and C. Radar, Elastomerics, September, 1987, p. 21) describe compositions that are mixtures of modified polyolefins (chlorinated polyolefins) and polyurethane resins. These compositions are reported to provide adhesion to a wide variety of other substrates. These compositions have several drawbacks, including insufficient adhesion and environmental resistance.
Modifications of polyolefins in order to promote adhesion to metal, glass, and other substrates are also well known in the art. Baum (U.S. Pat. No. 3,211,804) describes the modification of polyolefins with polymethylolated phenolic material containing one or more phenolic nuclei and having substituted on the phenolic nuclei at least two methylol groups (--CH.sub.2 OH). Baum further describes the adhesional benefits of carboxyl and hydroxyl groups attached to the polyolefin.
Van Meesche and Radar (A. Van Meesche and C. Radar, Elastomerics, September, 1987, p. 21) report that carboxylated polypropylene is an effective adhesive for bonding polyolefinic thermoplastic elastomers to metal, but it has the major disadvantage of a high activation temperature of .about.200.degree. C. Van der Kooi and Goettler (J. P. Van der Kooi and L. A. Goettler, Rubber World, Vol. 192, No. 2, 1985, p. 38) describe several modified polyolefins used to bond polyolefinic thermoplastic elastomers to metal, including carboxylated polyolefins. These polymers also have the disadvantage of a high activation temperature.
Nogues (U.S. Pat. No. 4,822,688) describes the use of non-halogenated polypropylene that has been grafted with maleic anhydride in adhesive formulations. Nogues further describes the reaction of such materials with polyols and polyamines. Bratawidjaja et al (Bratawidjaja, A. S.; Gitopadmoyo, I.; Watanabe, Y.; Hatakeyama, T. Journal of Applied Polymer Science, Vol. 37, 1989, p. 1141) also describe the relationship between the extent of grafting of maleic anhydride onto polypropylene and its adhesive strength to aluminum. These compositions have disadvantages that include high activation temperatures and insolubility in most solvents, precluding use in a solvent borne adhesive system.
Other methods of bonding polyolefins to substrates typically involve surface pretreatment of the polyolefin prior to bonding. These technologies involve such processes as plasma treatment, corona discharge, and chemical etching as with chromic acid. These technologies are widely known and practiced but have several disadvantages. The foremost of these disadvantages, with respect to bonding during molding of polyolefins such as polyolefinic thermoplastic elastomers, is that the polyolefin must have a solid surface to pretreat. When molding these materials onto a substrate, as in injection molding onto a metal insert, there is no polyolefin surface that is readily modified prior to adhesion. Thus the process of injection or compression molding effectively precludes surface pretreatment of the polyolefin to promote adhesion either to the substrate or to an adhesive already applied to the substrate.
Adhesives, such as described above, currently used for bonding polyolefinic thermoplastic elastomers to metal substrates have many drawbacks that are eliminated by the present invention. These drawbacks include the requirement for a primer applied to the substrate prior to the adhesive, poor shelf stability, poor environmental resistance, poor strength, inconvenient processing requirements, and toxic and/or flammable solvents.